In multiple input multiple output and multiple input single output systems (MIMO and MISO), Spatial Modulation is used. The fundamental component of Spatial Modulation is the exploitation of the spatial domain, i.e., the spatial position of the antenna at the transmitter-side, as a means for sending information through a wireless fading channel. In particular, the underlying principle of Spatial Modulation is twofold: i) at the transmitter-side, a one-to-one mapping of information data to transmit antennas, thus allowing them to convey information, and ii) at the receiver-side, the exploitation, thanks to the stochastic properties of wireless fading channels, of distinct multipath profiles received from different transmit antennas.
Contributions which are available for Spatial Modulation have been based on the same assumption: a uniform power allocation mechanism among the active transmit antennas is assumed a priori. The key problem of optimizing the effective spatial constellation pattern of Spatial Modulation has been addressed. However, the known optimization is based on the activation and de-activation of a set of antennas rather than on power allocation mechanisms. A uniform power allocation strategy is implicitly retained in the analysis.
Known solutions do also not fully exploit all degrees of freedom and potentialities of Spatial Modulation (SM) concept. In particular, the common limitation of all above techniques for SM is not taking maximum advantage of multiple antennas at the transmitter-side to obtain transmit-diversity gains. As a matter of fact, signal designs and optimal detectors available so far offer a diversity order that depends on the number of receive antennas only. As a consequence, SM methods proposed to date might find limited applicability to low-complexity and low-cost downlink settings and operations, where it is more economical to add equipment to base stations rather than to remote mobile units.
Spatial modulation is considered in: Y. Chau and S.-H. Yu, “Space shift keying modulation”, U.S. Pat. No. 9,985,988, Filed Nov. 7, 2001, Pub. Date Jul. 18, 2002; and
Y. A. Chau and S.-H. Yu, “Space modulation on wireless fading channels”, IEEE Vehicular Technology Conference—Fall, vol. 3, pp. 1668-1671, October 2001.
The disadvantages of such SM methods are as follows:
Even though two transmit-antennas are employed in the communication link, the SM proposal of Chau and Yu offers a diversity order only equal to one.
The error probability in depends only on the channel power gain of the wireless link related to the antenna that can be either switched on or off during data transmission. As a consequence, in an adaptive system and for optimizing the system performance, the antenna with the best (average) channel conditions may be chosen as the one to be switched on and off.
The SM concept introduced by Chau and Yu, which is called Space Shift Keying (SSK) in which only one transmit-antenna is activated when message 1 has to be sent, while both transmit-antennas are activated when a message 2 needs to be sent. Thus when message 2 has to be sent, each antenna at the transmitter-side is required to transmit a signal with a corresponding energy. This leads to a power consumptions cost which is twice with respect to that required when message 1 is sent.
Other SM methods are proposed in: C.-W. Ahn, S.-B. Yun, E.-S. Kim, H. Haas, R. Mesleh, T.-I. Hyon, and S. McLaughlin, “Spatial modulation method and transmitting and receiving apparatuses using the same in a multiple input multiple output system”, Filed Jul. 10, 2007, Pub. Date Feb. 14, 2008;
R. Y. Mesleh, H. Haas, S. Sinanovic, C. W. Ahn, and S. Yun, “Spatial modulation”, IEEE Transactions on Vehicular Technology, vol. 57, no. 4, pp. 2228-2241, July 2008; and
J. Jeganathan, A. Ghrayeb, and L. Szczecinski, “Spatial modulation: Optimal detection and performance analysis”, IEEE Communications Letters, vol. 12, no. 8, pp. 545-547, August 2008.
The disadvantages of such SM methods are as follows:
Even though two transmit-antennas are employed in the communication link, the SM proposal in Mesleh et al and Jeganathan et al offers, similar to Chau and Yu, a diversity order only equal to 1.
The error probability depends on both complex channel gains and, in particular, is a function of the difference of them. As a consequence, depending on the instantaneous channel conditions, constructive and destructive combinations can take place, thus preventing the full exploitation of the two transmit-antennas for diversity purposes.
The error probability is a function of the spatial correlation coefficient and, in particular, the more the wireless links are correlated, the worse the error probability is.